Imaging spectropolarimeters are employed in several fields including geology, where they are used in remote sensing to locate and identify aluminum, copper, iron, lead and quartz based on their polarized reflection spectrum. Conservationists have used imaging spectropolarimeters to map polarized solar reflection from water to aid in the delineation of wetlands. In addition, imaging spectropolarimeters are widely used in astronomy in traditional settings such as telescope-based instruments, as well as airborne, rocket-borne, and satellite platforms. Mapping polarization helps astronomers to determine what physical processes created the observed light. For instance, imaging spectropolarimeters are used to locate linearly and circularly polarized atomic transitions split by the Zeeman effect induced by the large magnetic fields present in nebulae and the corona of stars. Astronomers also use imaging spectropolarimeters to study clouds on our planet as well as others.
Most imaging spectrometers perform some form of scanning to measure a Spectral Object cube (SOC). Perhaps the simplest of imaging spectrometers is a camera (e.g., film, focal plane array (FPA), etc.) with a narrowband spectral filter. This device measures a horizontal planar region in the SOC. The filtered camera relies on swapping out filters of different color to fully record the SOC. Thus, this system is said to scan in wavelength, the third dimension of the SOC, to acquire the SOC.
The filtered camera has been improved upon with high speed electronic spectral filters such as acousto-optic tunable filters (AOTFs) that can rapidly change the filter color by changing the electronic drive signal to the AOTF. This procedure can be done in tens of milliseconds, but this is unacceptable for many dynamic situations where wavelength scanning is inappropriate.
Many other imaging spectrometers require scanning in a spatial domain to acquire the SOC. For instance, a slit from an imaging spectrometer is scanned across the scene in the direction perpendicular to the slit axis. This device measures a vertical planar structure in the SOC at any given instant. The slit image is dispersed perpendicular to the slit axis by a diffraction grating onto an FPA. The information on the FPA has spatial content along the slit axis and spectral content in the orthogonal direction.
Researchers at the University of Arizona developed the Computed Tomography Imaging Spectrometer (CTIS) which can measure the entire SOC in one FPA frame via two dimensional spectral dispersion and techniques using computed tomography. CTIS is the only known device that can measure the SOC without spatial or spectral scanning.
The Computed Tomography Imaging Spectropolarimeter (CTISP), developed by Dr. Miles for his dissertation, is a polarimetric extension to CTIS. CTISP measures the spectrally dependent polarization state of the light using the Stokes vector representation of polarized light. Stokes vectors have four elements, and thus one way of visualizing the CTISP measured information is to note that CTISP measures the four Stokes Spectral Object Cubes (SSOC). The alternative is visualization in seven dimensional space (1 wavelength, 2 spatial and 4 polarization). CTISP was a proof of principle instrument that demonstrated the concept of voxel polarimetric calibration. For all its merits, CTISP relied on scanning four discrete polarization analyzers to measure the SSOCs. Thus even CTISP, which did not have to scan in the spatial or spectral domains, required polarimetric scanning.
To further understand the merits of NS-CTISP it is instructive to compare it to previously developed spectropolarimeters, most of which were developed for astronomy.
In the 1980's the University of Wisconsin developed the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE) for the first ever exploration of astronomical UV polarization. WUPPE often operated as a linear polarimeter utilizing a pair of linear diode arrays to measure the Wollaston prism separated and then spectrally dispersed orthogonally polarized linear spectra. WUPPE was a competent polarimeter with an ability to measure all four Stokes parameters. For all its strengths, WUPPE provided no spatial resolution within the field of view; hence it was not an imaging device. Hence WUPPE measured a vertical column within each of the four Stokes object cubes similar to a whisk broom scanner.
The Wide-field Imaging Survey Polarimeter (WISP) was also developed by researchers at Wisconsin and is currently in use. This rocket borne device acquires wide field polarimetric images, but each image has a wide spectral band pass. WISP has two broadband filters centered at 164 and 218 nanometers. A quartet of CaF2 waveplates undergoes actuator induced stress birefringence to achieve ½ wave retardance for the wavelength of interest. Thus the device has good imaging capabilities, but it integrates the polarization response over two wide wavelength regions. WISP thus measures planar structures within each of the four Stokes cubes, much like a filtered camera.
Gary Schmidt and H. S. Stockman developed a CCD Imaging/Spectropolarimeter termed SPOL. The device can measure polarization in the full instrument spectral domain (380–900 nm) for a slit source with a spectral resolution from 0.6 to 1.2 nm. The device can operate as an imaging polarimeter if a spectral bandpass filter is used. Thus this device can perform spectropolarimetry of a slit source or imaging polarimetry of an extended source but not imaging spectropolarimetry of an extended source. SPOL can be configured to acquire single wavelength band planar structures in the Stokes object cubes or a single vertical planar structure when an entrance slit is used. In either case only data from a subset of the Stokes object cubes can be acquired without scanning.
The “Advanced Stokes Polarimeter” was developed by the National Center for Atmospheric Research and the National Solar Observatory. Again although a full Stokes vector is measured, this instrument provides polarimetric information from a slit source and not an extended source, thus its limitations are similar to SPOL's.
James Hansen of the Goddard Institute of Space Studies is developing a compact instrument to study particulates on Jupiter. This device performs photometry at seven narrow spectral bands and photopolarimetry at 410, 678 and 945 nm. This device utilizes several single element detectors and thus provides no imaging capability. The device then operates much like a whiskbroom system, but even with scanning only records several narrow band horizontal planar structures within the Stokes object cubes.
Lastly, Glenar et al. describe POLARIS-II, an imaging spectropolarimeter developed at Goddard Space Flight Center. Although this device is an imaging spectropolarimeter, it only provides linear polarization information and thus not a full Stokes vector.
Clearly, a wide array of capable instruments has been developed, but none are capable of full acquisition of all Stokes object cube data in one image with no spatial, spectral or polarization scanning required. Consequently, a significant need exists for a device and method for an imaging spectropolarimeter suitable for dynamic applications unsuitable for scanning.